1. Field of the Invention
The present invention relates generally to batteries for spacecraft applications and, more particularly, to a mass efficient spacecraft design without voltage regulation of the battery resulting in increased satellite reliability, lower mass, lower thermal load, lower bus voltage noise and EMI susceptibility, and lower cost. In one embodiment of the invention, the design and assembly of such batteries is particularly economical, yet tailored to meet the power requirements of specialized spacecraft. More specifically, according to this embodiment, a very large number of small battery cells are assembled electrically in parallel into a cell module to provide a larger ampere hour capacity and, in turn, multiple cell modules are series connected into a spacecraft battery having the required power. The invention provides for (a) three dimensional packaging of parallel cells, (b) rejecting the internal heat from such a package, and (c) providing redundant electrical fuse protection for the internal cell array. In short, the invention provides a unique methodology for utilizing mass produced electrochemical cells in low volume applications thereby reducing production and development costs and enables the provision of batteries with flexible capabilities for specialized applications.
2. Description of the Prior Art
As noted in the disclosure of application Ser. No. 08/955,931 filed Oct. 22, 1997, pending, satellite electrical power systems are classified as either regulated or unregulated. The difference is whether the power system voltage, when operated on the batteries, is regulated by a DC to DC converter or the satellite is powered directly by the battery In the latter case the satellite electrical loads must be designed to operate over the voltage range of the battery discharge which may include added radiator to compensate for thermal efficiency of the payload (it is not clear that this is a problem in eclipse battery operation). To maximize payload efficiency, the battery must operate over as narrow a voltage range as possible. This in turn requires for an unregulated bus that battery cells be redesigned when the satellite power is changed as the battery voltage must be fixed. Thus battery watt hour capacity can only be determined by the battery cell ampere-hour capacity.
As just noted, the regulated bus solves these problems through the DC to DC converter. This solution, however, comes at the price of added satellite weight and cost due to the converter, added battery weight in order to compensate for the discharge converted inefficiency, and possibly added thermal radiator to compensate for converter inefficiency.
As noted in the disclosure of application Ser. No. 08/955,931 filed Oct. 22, 1997, pending, at the present time, spacecraft batteries are typically built with custom fabricated cells built to match the required battery ampere hour capacity. This requires that new cell sizes be developed and manufactured as the required battery ampere hour capacity is changed. This in turn leads to specialized high cost, low volume, production facilities due to the limited cell quantities required for spacecraft use and the number of different sizes required. In addition, high quality is difficult to achieve in such production facilities as the quantities make it difficult to implement statistical process control and many of the manufacturing steps are carried out by hand. An alternative approach is to build batteries based on parallel arrays of smaller cells which may be commercially available, although this is not a requirement of the invention. These arrays are then connected in series to yield the required battery voltage and capacity. Difficulties with this approach are that (a) a short circuit will short out the entire parallel array, (b) such arrays are typically volumetrically inefficient due to the small height of a typical commercial cell compared with a larger custom design, and (c) if the cells are packaged in a three dimensional array for improved volumetric efficiency, waste heat cannot be efficiently removed from the interior of the cell array.
An alternative to respecifying ampere-hour cell capacity is to use a fixed capacity and DC to DC conversion. This approach, however, adds mass (by reason of the converter) and increases battery weight due to the inefficiency of the converter.
Additionally, it is customary for a known battery systems to incorporate appropriate electronics to automatically by-pass a mal-functioning cell. This electronics is costly, occupies limited space, and adds to the weight of the system.
Thus, it was determined to be desirable to assemble into a cell module a very large number of small battery cells, electrically in parallel, to provide a larger ampere hour capacity and, in turn, multiple cell modules are series connected into a spacecraft battery having the required power. That technique provides for (a) three dimensional packaging of parallel cells, (b) rejecting the internal heat from such a package, and (c) providing redundant electrical fuse protection for the internal cell array. Thus, at least first and second modules are provided, each being of plural commonly aligned, parallel bussed, electric cells, each cell including opposed ends and an outer peripheral surface. The cells are disposed in a planar side-by-side relationship, and each module includes a plurality of outermost cells encompassing an interior region. The modules are connected electrically in series and are laterally coextensive and aligned in end to end relationship. A heat sink is provided including a heat conductive body having a radiative surface facing deep space and heat shunts extend between the interior regions of the modules and the radiative surface. Fuse devices are provided in series with each of the electric cells and responsive to excessive current flow to sever connection of its associated said electric cell from said battery system.
Key elements of the technique recited in the disclosure of application Ser. No. 08/955,931 filed Oct. 22, 1997, pending, mentioned above, include the following:
Arraying small cylindrical or prismatic cells in parallel with multiple fuse protection. In one preferred embodiment for the invention, a large number of "18650" (that is, 18 mm in diameter, 650 mm long) lithium ion cells are bussed in parallel protected with one or more of the following three protective fuses: PA1 A three dimensional array of individual cells. The three dimensional array is achieved by bussing two dimensional parallel layers followed by stacking layers physically and parallel connecting individual layers; and PA1 Creating an internal thermal design inside the three dimensional array which shunts heat across individual layers to vertical thermal shunts which act to conduct heat to a spacecraft thermal radiator.
(i) a cell internal thermal fuse which irreversibly shuts down the cell in case of an internal short circuit; PA2 (ii) a cell automatically resetting thermal fuse which shuts down the cell in case of a transient high current short circuit; and PA2 (iii) an electrical fuse which is sized at between 1C and 10C current.
Building upon the foregoing, it is well known that satellite electrical power systems are classified as either regulated or unregulated. The difference is whether the power system voltage, when operated on the batteries, is regulated by a DC to DC converter or the satellite is powered directly by the battery. In the latter case the satellite electrical loads must be designed to operate over the voltage range of the battery discharge which may include added radiator to compensate for thermal efficiency of the payload (it is not clear that this is a problem in eclipse battery operation). To maximize payload efficiency (and in turn minimize any thermal and battery effects), the battery must be designed to discharge over a narrow voltage range. This in turn requires for an unregulated bus that battery cells be redesigned when the satellite power is changed as the battery voltage must be fixed. Thus battery watt hour capacity can only be determined by the battery cell ampere-hour capacity.
As earlier noted, the regulated bus solves these problems through the DC to DC converter This solution, however, comes at the price of added satellite weight and cost due to the converter, added battery weight in order to compensate for the discharge converted inefficiency, and possibly added thermal radiator to compensate for converter inefficiency.
It was with knowledge of the foregoing that the present invention has been conceived and is now reduced to practice.